Approche par modélisation pluie-débit de la connaissance régionale de la ressource en eau : application au haut bassin du fleuve Sénégal

De nos jours la gestion des ressources en eau est un besoin qui s’exprime à tous les niveaux (local, régional, national, internationale) du fait des conséquences souvent dramatiques d’une pénurie en eau sur le plan humain, économique et politique. Cependant, l’on ne peut bien gérer une ressource que quand elle est connue. Malheureusement les dernières décennies ont vu une réduction considérable des capacités nationales, des pays qui se partagent le bassin du fleuve Sénégal, d’assurer le suivi..

Approche par modélisation pluie-débit de la connaissance régionale de la ressource en eau : application au haut bassin du fleuve Sénégal

De nos jours la gestion des ressources en eau est un besoin qui s’exprime à tous les niveaux (local, régional, national, internationale) du fait des conséquences souvent dramatiques d’une pénurie en eau sur le plan humain, économique et politique. Cependant, l’on ne peut bien gérer une ressource que quand elle est connue. Malheureusement les dernières décennies ont vu une réduction considérable des capacités nationales, des pays qui se partagent le bassin du fleuve Sénégal, d’assurer le suivi..

Spatiotemporal Trend Analysis of the Mean Annual Rainfall in Senegal

The spatiotemporal trends of annual rainfall in Senegal during 1940 - 2013 were investigated using the Mann–Kendall test and Theil–Sen’s slope estimator. Theil and Sen's slope estimator test was used for finding the magnitude of change over a time period. Inverse Weight Distance (IDW) technique in Arc GIS 10.2 was used to investigate spatial patterns of the trends over the entire country. For the period 1940-2013, the results of the analysis showed negative trends in annual rainfall at the whole country except for the Bakel station which exhibits a positive trend but not significant. While for the period 1984 - 2013, all the stations show a positive trend with 07 out of 22 stations exhibiting a significant trend at the 95% confidence interval. The spatial distribution of trend during the period 1940- 2013 showed a significant negative trend in the whole study of area except small areas located at the extreme South Est and West as well as North East and West. The trend magnitude varies between -4.41mm/year to 1.34 mm for the period 1940-2013 with a maximum negative magnitude at the Tambacounda station. For 1984-2013, the trend magnitude is positive for the whole country with values varying between 2.67 mm/year at Goudiry and 12.2 mm/year at Ziguinchor. Magnitudes are greater than 5 mm/year, for stations with significant positive trend

Approche par modélisation pluie-débit de la connaissance régionale de la ressource en eau : application au haut bassin du fleuve Sénégal

De nos jours, la gestion des ressources en eau est un besoin qui s'exprime à tous les niveaux (local, régional, national, international) du fait des conséquences souvent dramatiques d'une pénurie en eau sur le plan humain, économique et politique. Cependant on ne peut bien gérer une ressource que quand elle est connue. Malheureusement les dernières décennies ont vu une réduction considérable des capacités nationales des pays qui se partagent le bassin du fleuve Sénégal, d'assurer le suivi hydrologique du fleuve et de ses affluents et de produire une information de qualité adaptée aux besoins des utilisateurs. Ainsi la connaissance de la ressource en eau et de ses variations saisonnières au niveau du haut bassin, de manière générale, et dans la partie guinéenne du bassin, en particulier, présente des faiblesses à cause des séries de données hydrologiques tronquées ou manquantes. Les chroniques de débit sont souvent lacunaires, discontinues, de courte durée et en conséquence sont difficilement exploitables pour une analyse hydrologique fiable. Dans ce contexte, l'objectif principal de cette étude est de reconstituer les données hydrométriques, surtout dans la partie guinéenne du bassin (ce pays vient de rejoindre l'OMVS en 2006, mais avec des données hydrométriques fragmentaires), en utilisant le modèle GR2M. Cela permet de disposer de séries chronologiques de débits assez longues pour une meilleure estimation des ressources en eau et de leur fluctuation temporelle. À cette fin, on a procédé dans un premier temps à une caractérisation physiographique, puis pluviométrique, du haut bassin. Ensuite, une modélisation hydrologique a permis de réaliser le calage/validation des bassins, grâce au modèle hydrologique GR2M, sur une période de référence. Enfin, nous avons évalué les impacts potentiels d'un changement climatique sur l'évolution des ressources en eau du haut bassin, en utilisant les sorties de quatre modèles climatiques (CSMK3, HADCM3, MPEH5 et NCPCM) issues du dernier rapport de IPCC 2007, sous influence du scénario SRES A2. La caractérisation du régime pluviométrique est basée sur une analyse statistique des pluies annuelles, mensuelles et journalières ; ensuite, une spatialisation des résultats des analyses statistiques a été effectuée. Ces cartes offrent un support de visualisation et de synthèse. Elles permettent de quitter la vision réduite à un point pour une vision globale de la pluie à l'échelle de la zone étudiée. Le calage/validation croisée du modèle a permis de choisir les paramètres qui traduisent le mieux la transformation de la pluie en débit. Une fois ces jeux de paramètres choisis pour chaque bassin, cette version du modèle a été appliquée sur l'ensemble de la série pour simuler les écoulements. Cette opération a permis d'étendre les séries de débits mensuels de trois bassins (Bafing Makana, Dakka Saidou et Sokotoro) de 1923 à 2005. Globalement, le modèle reproduit d'une manière satisfaisante la forme des hydrogrammes observés. Les débits de pointe calculés sont bien situés dans le temps, même s'ils sont parfois sous-estimés (particulièrement pour le bassin de Bafing Makana pour la période allant de 1981 à 1985) ou surestimés. Les étiages, par contre, sont parfaitement reconstitués et on observe une bonne superposition des hydrogrammes des débits observés et calculés. En raison de la variété des bassins (taille, physiographie, états de surface et caractéristiques des sols), qui leur donne une sensibilité différente aux différents termes du climat, les impacts d'un potentiel changement climatique sont variables d'un bassin à l'autre. Ainsi, par rapport aux données observées, les scénarios utilisés prévoient sur nos bassins des variations d'écoulement moyen annuel qui dépendent étroitement des prévisions de précipitation de chaque modèle climatique.Today water resources management is crucial at various scales (local, regional, national and international). This management is encouraged by the extreme hydrological events (droughts or floods) that can have dramatic consequences on human, economic and political aspects. Appropriately managing a resource requires its evaluation. However, over the last years, there was a tremendous decrease in hydrological monitoring capacities in the riverine countries of the Senegal River to produce sufficiently good information to meet end-users requirements on the main river and its tributaries. Thus there are some gaps in the knowledge of water resources and its seasonal variations in the upper Guinean basin due to discontinuous observation series. In this context, the main objective of this thesis was to simulate missing hydrological data using the GR2M model, especially in the upper Guinean basin. To this end, a physiographic and climatic characterization of the upper basin was performed. The pluviometric regime was characterized using a statistical analysis of annual, monthly and daily rainfall, with a spatial analysis of results. The produced maps provide a visualisation and synthesis tool, not only at the local scale but at the scale of the studied zone. Then the GR2M hydrological model was calibrated and validated on a reference period, which served for infilling gaps in monthly flow time series for the Bafing Makana, Dakka Saïdou and Sokotoro catchments over the 1923-2005 period. On average, the model satisfactorily reproduces the shape of the observed hydrographs. Last, we evaluated the potential impacts of climate change on the evolution of water resources on the upper basin using the outputs of four climate models (CSMK3, HADCM3, MPEH5 and NCPCM) from the 2007 IPPC runs based on the A2 SRES scenario. The impacts of climate change on flows differ between catchments due to different sensitivities to the various climatic variables. On the Bafing Makana catchment, an increase of flow is predicted by the year 2030 with then a decrease by the years 2060 and 2090 when using the CSMK3 and HADCM3 models. A continuous decrease is predicted until 2090 when using the MPEF5 model. Only low variations are predicted with the NCPCM model. At the Dakka Saidou station, the four models yield a decrease of flows for the three time horizons (2030, 2060 and 2090). The Sokotoro catchment is different from the two previous ones. On this catchment, an increase of flow is predicted when using the outputs of the CSMK3, HADCM3 and NCPCM models by the years 2030 and 2060. A decrease in predicted flows is obtained only with the MPEH5 model by the year 2060. For this catchment, a decrease of flows is simulated by the year 2090 compared to 2030 and 2060 when using the CSMK5, HADCM3 and MPEH5 models. A comparison of the annual variability of the outputs of the climate models shows that it is similar for the CSMK3, HADCM3 and MPEH5 models on the three catchments. They yield a progressive decrease in flows between 2030 and 2090. The NCPCM model is different and yields a progressive increase of flows between 2030 and 2090 for the three catchments. This model shows the lowest variability but is also the most optimistic in terms of future flows on the upper basin

Approche par modélisation pluie-débit de la connaissance régionale de la ressource en eau : application au haut bassin du fleuve Sénégal

Today water resources management is crucial at various scales (local, regional, national and international). This management is encouraged by the extreme hydrological events (droughts or floods) that can have dramatic consequences on human, economic and political aspects. Appropriately managing a resource requires its evaluation. However, over the last years, there was a tremendous decrease in hydrological monitoring capacities in the riverine countries of the Senegal River to produce sufficiently good information to meet end-users requirements on the main river and its tributaries. Thus there are some gaps in the knowledge of water resources and its seasonal variations in the upper Guinean basin due to discontinuous observation series. In this context, the main objective of this thesis was to simulate missing hydrological data using the GR2M model, especially in the upper Guinean basin. To this end, a physiographic and climatic characterization of the upper basin was performed. The pluviometric regime was characterized using a statistical analysis of annual, monthly and daily rainfall, with a spatial analysis of results. The produced maps provide a visualisation and synthesis tool, not only at the local scale but at the scale of the studied zone. Then the GR2M hydrological model was calibrated and validated on a reference period, which served for infilling gaps in monthly flow time series for the Bafing Makana, Dakka Saïdou and Sokotoro catchments over the 1923-2005 period. On average, the model satisfactorily reproduces the shape of the observed hydrographs. Last, we evaluated the potential impacts of climate change on the evolution of water resources on the upper basin using the outputs of four climate models (CSMK3, HADCM3, MPEH5 and NCPCM) from the 2007 IPPC runs based on the A2 SRES scenario. The impacts of climate change on flows differ between catchments due to different sensitivities to the various climatic variables. On the Bafing Makana catchment, an increase of flow is predicted by the year 2030 with then a decrease by the years 2060 and 2090 when using the CSMK3 and HADCM3 models. A continuous decrease is predicted until 2090 when using the MPEF5 model. Only low variations are predicted with the NCPCM model. At the Dakka Saidou station, the four models yield a decrease of flows for the three time horizons (2030, 2060 and 2090). The Sokotoro catchment is different from the two previous ones. On this catchment, an increase of flow is predicted when using the outputs of the CSMK3, HADCM3 and NCPCM models by the years 2030 and 2060. A decrease in predicted flows is obtained only with the MPEH5 model by the year 2060. For this catchment, a decrease of flows is simulated by the year 2090 compared to 2030 and 2060 when using the CSMK5, HADCM3 and MPEH5 models. A comparison of the annual variability of the outputs of the climate models shows that it is similar for the CSMK3, HADCM3 and MPEH5 models on the three catchments. They yield a progressive decrease in flows between 2030 and 2090. The NCPCM model is different and yields a progressive increase of flows between 2030 and 2090 for the three catchments. This model shows the lowest variability but is also the most optimistic in terms of future flows on the upper basin

Approche par modélisation pluie-débit de la connaissance régionale de la ressource en eau : application au haut bassin du fleuve Sénégal

Today water resources management is crucial at various scales (local, regional, national and international). This management is encouraged by the extreme hydrological events (droughts or floods) that can have dramatic consequences on human, economic and political aspects. Appropriately managing a resource requires its evaluation. However, over the last years, there was a tremendous decrease in hydrological monitoring capacities in the riverine countries of the Senegal River to produce sufficiently good information to meet end-users requirements on the main river and its tributaries. Thus there are some gaps in the knowledge of water resources and its seasonal variations in the upper Guinean basin due to discontinuous observation series. In this context, the main objective of this thesis was to simulate missing hydrological data using the GR2M model, especially in the upper Guinean basin. To this end, a physiographic and climatic characterization of the upper basin was performed. The pluviometric regime was characterized using a statistical analysis of annual, monthly and daily rainfall, with a spatial analysis of results. The produced maps provide a visualisation and synthesis tool, not only at the local scale but at the scale of the studied zone. Then the GR2M hydrological model was calibrated and validated on a reference period, which served for infilling gaps in monthly flow time series for the Bafing Makana, Dakka Saïdou and Sokotoro catchments over the 1923-2005 period. On average, the model satisfactorily reproduces the shape of the observed hydrographs. Last, we evaluated the potential impacts of climate change on the evolution of water resources on the upper basin using the outputs of four climate models (CSMK3, HADCM3, MPEH5 and NCPCM) from the 2007 IPPC runs based on the A2 SRES scenario. The impacts of climate change on flows differ between catchments due to different sensitivities to the various climatic variables. On the Bafing Makana catchment, an increase of flow is predicted by the year 2030 with then a decrease by the years 2060 and 2090 when using the CSMK3 and HADCM3 models. A continuous decrease is predicted until 2090 when using the MPEF5 model. Only low variations are predicted with the NCPCM model. At the Dakka Saidou station, the four models yield a decrease of flows for the three time horizons (2030, 2060 and 2090). The Sokotoro catchment is different from the two previous ones. On this catchment, an increase of flow is predicted when using the outputs of the CSMK3, HADCM3 and NCPCM models by the years 2030 and 2060. A decrease in predicted flows is obtained only with the MPEH5 model by the year 2060. For this catchment, a decrease of flows is simulated by the year 2090 compared to 2030 and 2060 when using the CSMK5, HADCM3 and MPEH5 models. A comparison of the annual variability of the outputs of the climate models shows that it is similar for the CSMK3, HADCM3 and MPEH5 models on the three catchments. They yield a progressive decrease in flows between 2030 and 2090. The NCPCM model is different and yields a progressive increase of flows between 2030 and 2090 for the three catchments. This model shows the lowest variability but is also the most optimistic in terms of future flows on the upper basin

Apport de la modélisation pluie-débit pour la connaissance de la ressource en eau : application au haut Bassin du Fleuve Sénégal

Les chroniques de débit du haut bassin du fleuve Sénégal et de sa partie guinéenne sont souvent lacunaires, discontinues, de courte durée, et donc difficilement exploitables pour une analyse hydrologique fiable. L’objectif de ce travail est de simuler puis d’étendre, dans la mesure du possible, les séries hydrologiques en utilisant le modèle pluie-débit, au pas de temps mensuel, GR2M. Dans un premier temps, après avoir calculé les pluies de bassin selon trois méthodes d’interpolation (krigeage, fonction spline et distance inverse pondérée au carré), l’ETP moyenne et la capacité en eau maximale, minimale et moyenne du sol des bassins (ou Water Holding Capacity - WHC), nous avons effectué une analyse de sensibilité du modèle aux données d’entrée pluie et WHC. Ceci a permis de choisir les meilleures combinaisons de données d’entrée par bassin en fonction du critère de Nash-Sutcliffe. Nous avons ensuite procédé au calage et à la validation croisée du modèle avec les données d’entrée choisies afin de déterminer les jeux de paramètres du modèle qui traduisent le mieux la transformation de la pluie en débit. Une fois le jeu de paramètres choisi pour chaque bassin, nous avons appliqué cette version du modèle sur l’ensemble de la série pluviométrique disponible pour simuler les débits. Cette méthodologie a permis de compléter et d’étendre les séries de débits mensuels des bassins versants de Bafing Makana, Dakka Saidou et Sokotoro de 1960 à 2000

Recent hydrological evolutions of the Senegal River flood (West Africa)

The flood of the Senegal River is a resource for the populations of the Senegal Valley, who practice agriculture, fishing and livestock farming in the annually flooded areas. The objective of this paper is to investigate the hydrological evolution of the Senegal River flood since the drought of the 1970s and the construction and management of the Manantali Dam. To this purpose, the paper analyses the evolution of several parameters of the flood hydrograph and defines the factors that determine the intensity of the flood. Also, a follow-up of the flooded areas in the valley is carried out using remote sensing. The results show that the flood hydrograph has been similar since the 1970s drought, due to the reduction in contributions from the Bafing after the construction of the Manantali Dam. As a result, the high floods depend now on the rainfall pattern in the Sudano-Sahelian zone of the watershed

Testing sensitivity of BILAN and GR2M models to climate conditions in the Gambia River Basin

This study investigates the performance of two lumped hydrological models, BILAN and GR2M, in simulating runoff across six catchments in the Gambia River Basin (Senegal) over a 30-year period employing a 7-year sliding window under different climatic conditions. The results revealed differences in overall performance and variable sensitivity of the models to hydrological conditions and calibration period lengths, stemming from their different structure and complexity. In particular, the BILAN model, which is based on a more complex set of parameters, showed better overall results in simulating dry conditions, while the GR2M model had superior performance in wet conditions. The study emphasized the importance of the length of the calibration period on model performance and on the reduction of uncertainty in the results. Extended calibration periods for both models narrowed the range of the Kling-Gupta Efficiency (KGE) values and reduced the loss of performance during the parameter transfer from calibration to validation. For the BILAN model, a longer calibration period also significantly reduced the variability of performance metric values. Conversely, for the GR2M model, the variability rate did not decrease with the length of the calibration periods. Testing both models under variable conditions underscored the crucial role of comprehending model structure, hydrological sensitivity, and calibration strategy effects on simulation accuracy and uncertainty for reliable results

Paradoxe d'évaporation dans la vallée du fleuve Sénégal

The increase in temperatures in a context of climate change should be accompanied by an increase in evaporation or evapotranspiration (ET0). However, studies have shown a decrease in evaporation (or evapotranspiration) in certain regions of the world. This contrast between the decrease in evaporation and the increase in temperature, which varies according to the climatic zones, is known as the "evaporation paradox". The objective of this work is to examine the causes in the climatic context of the Senegal River Valley. The climatic data (evaporation, rain, temperature, relative humidity, insolation) observed at the stations of Bakel, Podor, Matam and Saint-Louis were used over the period 1981-2015. The methodology includes: 1/ the detection of ruptures by the PETTITT test, 2/ the analysis of the relationship between evaporation / evapotranspiration and climatic variables by the PEARSON correlation test, 3/ the detection of trends in Evaporation / ET0 and climate variables by the MANN-KENDALL test and SEN’s slope. The breaks in evaporation and evapotranspiration are respectively identified in the 1990s and 2000s. The PEARSON correlation shows that evaporation and ET0 have their strongest relationship with the relative humidity. MANN-KENDALL's test reveals a significant decrease (p-value ˂ 0.001) in evaporation and a mixed trend (decrease and increase) in reference evapotranspiration. This decrease in evaporation, combined with a significant increase in temperature, highlights the existence of an evaporation paradox in the Senegal river valley. This evaporation paradox is explained here by the significant increase in relative humidity and rainfall, but also by the significant decrease in sunshine hours